The present invention relates to a radio communication method and a radio communication device for transmitting a burst signal including a synchronization identification signal and a data signal during communication.
TDD/TDMA (Time Division Duplex/Time Division Multiple Access) is known as one of typical radio communication methods used for example in PHS (Personal Handy-phone system). TDD switches the same radio path to an uplink and a downlink frequently along the time axis. TDMA allows several users to share the same frequency channel by dividing the signal into different time slots.
In TDMA, a burst signal is transmitted from a radio communication terminal to a base station intermittently. The burst signal includes from its top a synchronization identification signal (also referred to as “preamble”), data format signal and data signal (also referred to as “burst data”). The synchronization identification signal is a signal mainly for keeping data synchronization to recover data. The data format signal is a signal for indicating a modulation method for the subsequent data signal. More specifically, in the case of PHS or EVDO using CDMA, unique word and pilot symbol correspond to symbol for synchronization, modulation information corresponds to the data format, and signal data symbol corresponds to the data signal.
There are modulation methods for data signal such as BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 16QAM (Quadrature Amplitude Modulation) and 64QAM. An error correction technique such as FEC (Forward Error Correction) is used in those modulation methods.
The reachable distance of a radio wave broadcasted from a base station to terminals is defined by a value “link budget” (dB). The larger a link budget becomes, the bigger a cell of radio wave formed by a base station becomes. Therefore, communication in a wide area can be served by a less number of base stations.
A link budget is improved when an absolute power of radio wave is increased by increasing a transmit power of the radio wave or by intensifying antenna gain. A link budget is improved also when the Line of Site (LOS) condition is improved and reaching power between a base station and terminals is increased, by locating a base station at a higher position.
Patent Document 1 (Japan Laid Open Patent H10-336104) discloses a prior art in which the transmission power level of radio wave from a mobile station is measured by a receiver, and based on the measured transmission power level the output power level of a transmitter is controlled. In Patent Document 1, it is determined how much output power level of radio wave is appropriate for the mobile station to receive radio wave of a certain intensity of electric field. Then, based on the level of received radio wave, the output power level of the transmitter is controlled in the time slot for communicating with the mobile station.
However, there is an increasing limit to the transmit power since an effective transmit power thereof is restricted by a communication standard or a communication law. In addition, Patent Document 1 is for reducing radio wave interference generated between base stations by controlling the output power level so that it works exclusively for reducing a radio wave reachable range. So, Patent Document 1 cannot be used for improving the link budget.
It is easy way to arrange an antenna on the top of a building for locating a base station at a higher position. However, there is not always a high building around a place where an antenna should be arranged. Even if there is such a high building, it is not always allowed to arrange an antenna in the building. Therefore, it is not a reliable way to use a high building for the purpose.
As argued above, an alternative way to improve the link budget should be considered.    Patent Document 1: Japan Laid Open Patent H10-336104
In order to improve the link budget, the inventor paid attention to the fact that if radio communication by lower effective transmit power were possible, a base station could communicate with a mobile station more distant from it i.e. the link budget could be improved.
In order to make a communication possible even by lower effective transmit power, SN (Signal/Noise) ratio should be raised. For this purpose, the error correction technique FEC can be used for improving communication quality i.e. error tolerance. However, the synchronization identification signal does not conform to FEC. If FEC process were applied to the synchronization identification signal, the communication would be terminated. Accordingly, FEC can be applied only to the data signal. The error tolerance of the data signal part of a burst signal can be improved when applying FEC to the data signal part, but the error tolerance of the synchronization identification signal part cannot be improved since FEC cannot be applied to the synchronization identification signal part.
FIG. 7 shows a relationship between conventional effective transmit power and error rate. In FIG. 7, synchronization identification signals are those of BPSK and QPSK and data signals are those of FEC-BPSK and FEC-QPSK to which FEC has been applied.
As known from FIG. 7, in any one of the communication methods, the higher effective transmit power becomes, the lower error rate such as frame error rate becomes. It means that the error rate is improved. Error rates of the FEC-applied data signals are improved linearly and rapidly as the effective transmit power is increased. On the other hand, error rates of the synchronization identification signals of BPSK and QPSK are not so improved even if the effective transmit power is increased. At the error rate threshold i.e. communication limit, there is a large difference w0 in the effective transmit power, for example between BPSK and FEC-BPSK. That is to say, error tolerance difference is generated. As apparent from this, the link budget improvement of whole system reaches the limit in the part without FEC.
So, the object of the present invention is to provide a radio communication method and a radio communication device which can improve the link budget without increasing the effective transmit power in order to enlarge a size of a cell formed by a base station.
In order to solve the above stated problems, according to the present invention, a radio communication method for transmitting a burst signal including a synchronization identification signal and a data signal comprising: a step for comparing the error tolerances of the synchronization identification signal and the data signal; and a step for varying transmission power(s) of one or both of the synchronization identification signal and the data signal in accordance with the error tolerances compared.
According to the method above, the transmission powers of the synchronization identification signal and the data signal in the burst signal can be controlled separately in accordance with their error tolerances. Therefore, radio communication with an appropriate transmission power level can be carried out.
In said step for varying transmission power(s), it is preferable to increase the transmission power of one of the signals, the synchronization identification signal or the data signal, with a lower error tolerance. According to this, the error rate of only one of the signals with a higher error rate can be decreased so that the link budget can be improved.
In said step for varying transmission power (s), it is also preferable to decrease the transmission power of one of the signals, the synchronization identification signal or the data signal, with a higher error tolerance. According to this, the transmission power of one of the signals with a lower error rate can be reduced so that an efficient communication is achieved.
In said step for varying transmission power (s) of one or both of the synchronization identification signal and the data signal, the effective transmit power of the whole burst signal may be an almost fixed value and the effective transmit power of one of the signals is increased and the effective transmit power of the other one is decreased. According to this, the link budget of the whole system can be improved without increasing the effective transmit power.
The transmission power of the synchronization identification signal or of the data signal may be varied in accordance with an error tolerance determined based on the modulation method for the data signal and synchronization identification signal. The error rate may be actually measured during communication, but if a modulation method is decided, the error rate of an effective transmit power is also decided by calculation or by experiences. Accordingly, the amount of transmission power variation can also be determined. In other words, without taking care of the error rate, the amount of variation of the effective transmit power can be determined only from the modulation method in order to control transmitting power.
According to the present invention, a radio communication device for radio-communicating with a base station or with a radio communication terminal comprising: a communication unit for transmitting or receiving a burst signal including a synchronization identification signal and a data signal; an error tolerance comparison unit for comparing error tolerances of the synchronization identification signal and of the data signal included in the burst signal; and a transmitting power control unit for controlling the output power of said communication unit; wherein said transmitting power control unit varies transmission power(s) of one or both of the synchronization identification signal and the data signal in the burst signal in accordance with the comparison result of said error tolerance comparison unit.
According to the above structure of the radio communication device of the invention, the synchronization identification signal and the data signal in the burst signal can be transmission power-controlled separately in accordance with their error tolerances. Therefore, radio communication with an appropriate transmission power level can be carried out.